A shaped charge is an explosive device in which a metal shell called a liner, often conical or hemispherical, is surrounded by a high explosive charge, enclosed in a steel case. When the explosive is detonated, the liner is ejected as a very high velocity jet that has great penetrative power. The study of penetration by a shaped charge jet is of great importance, in respect of both military and civil applications. The latter include the oil industry, ejector seat mechanisms, and also civil engineering work such as the decommissioning of large structures.
Early work on shaped charges showed that a range of alternative constructions, including modifying the angle of the liner or varying its thickness, would result in a faster and longer metal jet. These research and development efforts to maximize penetration capabilities were based largely on trial and error. It was not until the 1970s that modeling codes could predict with any accuracy how a shaped charge would behave. While the concept of a metal surface being squeezed forward may seem relatively straightforward, the physics of shaped charges is very complex and even today is not completely understood.
One field that has benefited greatly from the use of shaped charges is the production of oil and gas. Oil and gas is located in subterranean formations. These formations have a permeability that dictates the rate at which the oil or gas can flow through the formation. To improve this permeability, the formation can be fractured.
Before fracturing occurs, a well is bored into the formation. Individual lengths of relatively large diameter metal tubulars are secured together to form a casing string that is positioned within a subterranean well bore to increase the integrity of the well bore and provide a path for producing fluids from the formation to the surface. Conventionally, the casing is cemented to the well bore face and subsequently perforated by detonating shaped explosive charges. These perforations extend through the casing and cement a short distance into the formation. In certain instances, it is desirable to conduct such perforating operations with the pressure in the well being overbalanced with respect to the formation pressure. Under overbalanced conditions, the well pressure exceeds the pressure at which the formation will fracture, and therefore, hydraulic fracturing occurs in the vicinity of the perforations. As an example, the perforations may penetrate several inches into the formation, and the fracture network may extend several feet into the formation. Thus, an enlarged conduit can be created for fluid flow between the formation and the well, and well productivity may be significantly increased by deliberately inducing fractures at the perforations.
When the perforating process is complete, the pressure within the well is allowed to decrease to the desired operating pressure for fluid production. As the pressure decreases, the newly created fractures tend to close under the overburden pressure. To ensure that fractures and perforations remain open conduits for fluids flowing from the formation into to the well or from the well into the formation, particulate material or proppants are conventionally injected into the perforations so as to prop the fractures open. In addition, the particulate material or proppant may scour the surface of the perforations and/or the fractures, thereby enlarging the conduits created for enhanced fluid flow. The proppant can be emplaced either simultaneously with formation of the perforations or at a later time by any of a variety of methods.
As the high-pressure pumps necessary to achieve an overbalanced condition in a well bore are relatively expensive and time consuming to operate, propellants have been utilized in conjunction with perforating techniques as a less expensive alternative to hydraulic fracturing. Shaped explosive charges are detonated to form perforations that extend through the casing and into the subterranean formation and a propellant is ignited. The gas generated by the burning (deflagration) of the propellant pressurizes the perforated subterranean interval and initiates and propagates fractures therein.
U.S. Pat. Nos. 4,633,951, 4,683,943 and 4,823,875 to Hill et al. describe a method of fracturing subterranean oil and gas producing formations wherein one or more gas generating and perforating devices are positioned at a selected depth in a wellbore by means of a wireline that may also be a consumable electrical signal transmitting cable or an ignition cord type fuse. The gas generating and perforating device is comprised of a plurality of generator sections. The center section includes a plurality of axially spaced and radially directed perforating shaped charges that are interconnected by a fast burning fuse. Each gas generator section includes a cylindrical thin walled outer canister member. Each gas generator section is provided with a substantially solid mass of gas generating propellant which may include, if necessary, a fast burn ring disposed adjacent to the canister member and a relatively slow burn core portion within the confines of ring. An elongated bore is also provided through which the wireline, electrical conductor wire or fuse that leads to the center or perforating charge section may be extended. Detonating cord fuses or similar igniters are disposed near the circumference of the canister members. Each gas generator section is simultaneously ignited to generate combustion gasses and perforate the well casing. The casing is perforated to form apertures while generation of gas commences virtually simultaneously. Detonation of the perforating shaped charges occurs at approximately 110 milliseconds after ignition of gas generating unit and that from a period of about 110 milliseconds to 200 milliseconds a substantial portion of the total flow through the perforations is gas generated by gas generating unit. None of these devices made use of a propellant to increase the effectiveness of the shaped charge.
U.S. Pat. No. 5,775,426 to Snider et al. provides one example of an improved shaped charge that uses a propellant. FIG. 1 illustrates the concept behind the Snider et al. apparatus 100. The shaped charge is located in case 110. It is mounted in a cylindrical carrier 122. A propellant sleeve 120 is located around the carrier. Propellant sleeve 120 may be cut from a length of propellant tubular and positioned around perforating charge carrier 122 at the well site. The apparatus 100 is then located in the well with the perforating charges adjacent the formation interval to be perforated. The perforating charges 110 are then detonated. Upon detonation, each perforating charge 110 blasts through a scallop 124 in carrier 122, penetrates propellant sleeve 120, creates an opening in casing 102 and penetrates formation forming perforations therein. Propellant sleeve 120 breaks apart and ignites due to the shock, heat, and pressure of the detonated shaped charge 110. When one or more perforating charges penetrate the formation, pressurized gas generated from the burning of propellant sleeve 120 enters formation 104 through the recently formed perforations thereby cleaning such perforations of debris. These propellant gases also stimulate formation 104 by extending the connectivity of formation 104 with the well by means of the pressure of the propellant gases fracturing the formation.
A standard perforating shaped charge 110 is shown in FIG. 1B. It includes a charge case 112, typically steel or zinc, a booster 114, and an explosive 116 also known as the main load, along with a metal liner 118.
One drawback of the Snider et al. device is that it requires a substantial volume of well fluid to be placed above the device prior to ignition. This fluid provides the initial hydrostatic pressure required to facilitate the desired propellant burn rate after ignition. In other words, the burn rate is proportional to the hydrostatic pressure. The fluid also enables temporary confinement of the gas pressure generated by burning of the propellant. Basically, the well fluid prevents the combustion gas from escaping up the well bore, resulting in the build-up of the gas pressures required to fracture the formation rock. However, this also means that a great deal of the energy created by the propellant is lost on the well fluid instead of the formation. The efficiency of the Snider et al. device is directly controlled by the amount and type of well fluid.
FIG. 2 provides an illustration of another shaped charge as disclosed in published U.S. Patent Application No. 2003/0037692 to Liu. In one embodiment 200 of the Liu device, he uses a liner having two layers, a high-density airside layer 202 and a low-density explosive side layer 204. Layer 202 can be made of high-density compositions like iron, tin, copper, tungsten, lead etc., in solid alloy or in compacted powder form, as is used in conventional deep penetration shaped charges. The explosive-side layer 204 can be made of solid aluminum or compacted aluminum powder. The explosive 206 is a mixture of high explosive and aluminum (HE/Al) with surplus aluminum (Al) in stoichiometry. The charge penetrates the target and releases a substantial amount of Al in molten state, inducing an Al—H2O reaction in water. Thus, Liu uses aluminum in both the explosive and as a propellant layer. And while the aluminum is effective in the presence of water, this technique fails if the aluminum is too cool (below 660° C.) or if there is insufficient quantities of water in the formation or in the gaseous, explosive combustion by-products. Also, the burn rate of the aluminum is not as variable and controllable as needed to fracture various types of rocks under varying over-burden stress conditions.
Despite the advances of Snider and Liu, a need still exists for a shaped charge that combines the variable burn rate and long burn time of the Snider device with Liu's combination shaped charge that both penetrates and fractures the rock.